Answers:

How to Restore Shine of Ceramic Tiles

Ceramic tiles have a glaze on them. This is virtually indestructible. Do your tiles have a film on them from hard water or soap scum or ?. I am guessing that this is what the problem is and so am suggesting vinegar to remove the hard water film. If it is a build-up in the shower you may want to get one of the products made specifically for this type of cleaning.